Conventionally, precipitated silica has mainly been used as a white reinforcing filler in rubber compositions. However, a rubber composition into which precipitated silica is blended has problems as described in the following, as compared with other white fillers: (1) the viscosity of the rubber which has not been vulcanized is high; (2) the vulcanizing speed becomes small; (3) the precipitated silica is not easily dispersed since the silica has a high self-aggregating property; and (4) the rubber elasticity is damaged although the reinforcing performance becomes high.
As a means for solving such problems, attempts of incorporating various vulcanization promoters, such as a silane coupling agent, are made (Patent Documents 1 and 2).
Usually, a silane coupling agent is in a liquid form at ordinary temperature. However, the agent is not easily mixed with rubber which is the partner material therewith since the rubber is in a solid form. Moreover, the agent is not uniformly dispersed with ease since the amount thereof added to the rubber is relatively small.
Known is a rubber composition wherein a silane coupling agent is supported on carbon black at an amount of about 50%, thereby improving the workability and reinforcing performance, in order to make the handling easy and improve the dispersibility (Patent Document 3). However, the composition is unsuitable for colored rubber products since the rubber is colored into black.
It can be supposed that a silane coupling agent is supported on a white filler such as calcium carbonate (Patent Document 4), or precipitated silica or hydrated calcium silicate (Patent Document 5) in order to improve the designability. However, since the water content in precipitated silica is high, it is feared that the silane coupling agent is inactivated. Thus, there arise problems regarding the storage stability and the storage control. Conventional calcium carbonate cannot be made into a high concentration since the calcium carbonate exhibits a small oil absorption. Moreover, the calcium carbonate has a problem regarding the reinforcing performance since particles of the calcium carbonate are not smaller than those of carbon black or precipitated silica.
Patent Document 1: Japanese Patent Application Laid Open (JP-A) No. 2003-155380
Patent Document 2: JP-A No. 2003-192842
Patent Document 3: JP-A No. 54-68860
Patent Document 4: JP-A No. 56-104950
Patent Document 5: JP-A No. 51-23540